Exhaust systems for internal combustion engines are conventionally built of components through which exhaust gas flows, on the whole, in all operating situations and together form the exhaust system. These components may be, in addition to one or more line sections, for example, one or more turbo chargers, one or more catalytic converters and/or one or more mufflers. Air correspondingly flows through exhaust systems for internal combustion engines in all operating situations and exhaust systems usually have one or more filters, valves and compressors.
Exhaust systems and intake systems have recently been started to be complemented by systems for actively influencing noise carried in the exhaust system or intake system, which can be attributed to the operation of an internal combustion engine. Such systems superimpose the noise, which is being carried in the exhaust system or intake system and is generated essentially by the internal combustion engine, with artificially generated sound waves, which muffle the noise being carried in the exhaust system or intake system. As a result, a sound released to the outside of the exhaust system or intake system shall fit the image of a particular manufacturer, appeal to customers and comply with legally required limit values.
This is achieved by at least one sound generator being provided, which is in fluidic connection with the exhaust system or intake system and thus radiates sound into the interior of the exhaust system or intake system. This artificially generated sound and the sound generated by the internal combustion engine are superimposed to one another and leave the exhaust system or the intake system together. Such systems may also be used for muffling. To achieve a complete destructive interference of the waves of the noise being carried in the exhaust system or intake system and of the sound generated by the sound generator, the sound waves originating from the loudspeaker must correspond in terms of amplitude and frequency to the sound waves being carried in the exhaust system or intake system, but have a phase shift of 180° relative to these. Even if sound waves, which are being carried in the exhaust system or intake system and can be attributed to the operation of the internal combustion engine, and the sound waves generated by the loudspeaker correspond to each other in terms of frequency and have a phase shift of 180° relative to each other, but the sound waves do not correspond to each other in terms of the amplitude, there will only be an attenuation of the noise emitted from the exhaust system or intake system.
An exhaust system with a system for actively influencing sound being carried in the exhaust system from the state of the art will be described below with reference to FIGS. 1 and 2.
An exhaust system 4 with a system 7 for actively influencing sound being carried in the exhaust system 4 has a sound generator 3 in the form of a sound-insulated housing, which contains a loudspeaker 2 and is connected to the exhaust system 4 in the area of a tail pipe 1 via a sound line. The tail pipe 1 has an orifice 8, which releases exhaust gas being carried in the exhaust system 4 and airborne sound being carried in the exhaust system 4 to the outside. An error microphone 5 is provided at the tail pipe 1. The error microphone 5 measures sound in the interior of the tail pipe 1. This measurement by means of the error microphone 5 takes place in a section located downstream of an area in which the sound line opens into the exhaust system 4 and the fluidic connection is thus established between the exhaust system 4 and the sound generator 3. The term “downstream” is related here to the direction of flow of the exhaust gas in the tail pipe 1 of the exhaust system 4. The direction of flow of the exhaust gas is indicated by arrows in FIG. 2. Additional components of the exhaust system 4, for example, a catalytic converter and a muffler, may be provided (not shown) between the area of the fluidic connection between the exhaust system 4 and the sound generator 3 and the internal combustion engine 6. The loudspeaker 2 and the error microphone 5 are connected each to a control 9. Further, the control 9 is connected to an engine control 6′ of an internal combustion engine 6 via a CAN bus. The internal combustion engine 6 further has an intake system 6″. Based on sound measured by the error microphone 5 and operating parameters of the internal combustion engine 6, which are received via the CAN bus, the control 9 calculates for the loudspeaker 2 a signal, which generates a desired overall noise when superimposed to the sound being carried in the interior of the tail pipe 1 of the exhaust system 4, and emits this at the loudspeaker 2. The control may use, for example, a filtered-x least mean squares (FxLMS) algorithm and attempt to control a feedback signal/error signal measured by means of the error microphone to zero by outputting sound via the loudspeaker (in case of sound cancellation) or to control a predefined threshold value (in case of sound influencing). Another bus system may also be used instead of a CAN bus.
The mode of operation of the control will be explained in more detail below with reference to FIGS. 3 through 5 based on the example of an active noise cancellation (ANC) control.
Many noises, which are generated by machines, for example, internal combustion engines, compressors or propellers, have periodic components. By monitoring the machine in question with a suitable sensor (e.g., tachometer), this makes it possible to provide a time-dependent input wave vector x(n), which has a dependence on the basic frequency and the harmonics of the noise predominantly by the machine. For example, the exhaust gas back pressure, the mass flow of the exhaust gas, the temperature of the exhaust gas, etc., may be involved in this dependence. Many machines generate noises of different basic frequencies; these are often called engine harmonics.
This time-dependent input wave vector x(n) has, as is shown in FIG. 3, an influence on the signal generated by the noise source according to an unknown z-transformed transfer function of the noise source P(z) (the signal corresponding to generated noise to be superimposed), d(n), and is used by the control algorithm of a system for actively influencing sound (called “ANC core” in FIGS. 3, 4A, 4B and 6A) for generating a sound, which corresponds to a sound corresponding to a signal u(n) used for the superimposition, which sound leads to a desired noise corresponding to the feedback signal e(n) when superimposed with the sound corresponding to the signal d(n) to be superimposed. The signal u(n) used for the superimposition corresponds (within the operating range) to the sound pressure of a sound generator, which generates the sound to be superimposed to the sound pressure of a sound generator. The transfer function of the source Pz can be determined empirically.
The superimposition is symbolized in FIG. 3 by the summation sign Σ and takes place in the acoustic area (e.g., in an exhaust gas line). The feedback signal e(n) arising from the superimposition is detected, for example, by means of an error microphone and returned to the control algorithm (ANC core) as a feedback signal.e(n)=d(n)−u(n).The feedback signal e(n) thus corresponds to a sound pressure of the superimposed noise.
In FIG. 3, P(z) is the Z-transform of the transfer function of the noise source. This transfer function P(z) may depend, besides on the basic variable of the machine generating the noise (in this case a time-dependent input wave vector x(n) representing the speed of rotation), on numerous physical parameters, for example, pressure, mass flow rate and temperature of the sound-carrying system. The transfer function of the noise source P(z) is not, as a rule, known exactly and is often determined empirically.
It is known that the model of the ANC control shown in FIG. 3 has shortcomings, because the feedback signal e(n), which is returned to the control algorithm and is obtained from the superimposition of the signal d(n) to be superimposed, which is generated by the noise source on the basis of the transfer function of the noise source P(z), to the sound generated by the sound generator corresponding to the signal u(n) used for the superimposition, contains components that cannot be attributed to the transfer function P(z) of the noise source.
The model of the ANC control is subsequently expanded by a transfer function of the sound generator S(z), as is shown in FIGS. 4A and 4B.
This transfer function of the sound generator S(z) takes into account, on the one hand, shortcomings of the digital-analog (D/A) converters, filters, amplifiers, sound generators, etc., used in the electrical field, but also of the path from the site of the sound generation/sound superimposition to the site of an error microphone determining the feedback signal e(n), which path is not yet taken into account in the acoustic area by the transfer function of the noise source P(z), and, finally, shortcomings of the error microphone, preamplifier, anti-aliasing filter and analog-digital (A/D) converter, etc., adjoining this in the electrical area.
In expanding the model from FIG. 3, the signal y(n) outputted by the ANC core therefore takes into account in the model according to FIGS. 4A and 4B the transfer function of the sound generator S(z), which is involved in the conversion of the signal y(n) outputted by the ANC core into the signal u(n). The signal u(n) used for the superimposition corresponds in this case to the (mathematically idealized) amplitude of the signal generated by the sound generator.
The transfer function of the sound generator, S(z), takes into account the entire area from the output of the control to the feedback signal of the control.
When noises are generated by the noise source (i.e., the noise source is switched on), the transfer function of the sound generator S(z) is obtained asS(z)=u(z)/y(z)and the signal u(n) used for the superimposition corresponds to the convolution of the signals s(n) and y(n)u(n)=conv[s(n), y(n)],wherein s(n) is the pulse response of the transfer function of the sound generator S(z). e(z), y(z) and u(z) are the respective Z-transforms of the signals e(n), y(n) and u(n).
FIG. 4B shows the model from FIG. 4A in more detail. As can be seen, the signal y(n) outputted by the ANC core is composed of two sinusoidal oscillations sin(ω0n), cos(ω0n), which are provided by a sine wave generator, are shifted by 90° relative to one another and are amplified before by different gain factors w 1(n), w2(n) by means of two amplifiers in order to generate two signals y1(n), y2(n) shifted by 90° relative to one another with different amplitudes. The gain of the two amplifiers is correspondingly adapted dynamically by an adaptation circuit as a function of the feedback signal e(n).
If, for example, the ith engine harmonic EOi shall be cancelled for a certain speed of rotation RPM of the internal combustion engine, the basic frequency f0 to be cancelled is obtained asf0=EOi·RPM/60,ω0=2πf0.The adaptation circuit used to adapt the gain in FIG. 4B is operated with a clock frequency that sets the clock frequency of the ANC core.
FIG. 5 schematically shows the spectral profile of the amplitude (Magn) of the noise (noise(n)) over the frequency (Freq). The signal d(n) to be superimposed indicates here the current sound pressure at the given basic frequency f0 in Pascals. ∥d(f)∥ shows the value of the amplitude at a defined time for harmonics.
The input wave vector x(n) of the ANC control is defined now as follows (vectors are printed in bold):x(n)=[sin(ω0n), cos(ω0n)].It was demonstrated in the paper “Active Noise Control: A tutorial review” by Sen M. Kuo and Dennis R. Morgan, published in the Proceedings of the IEEE, Vol. 87, No. 6, June 1999, that the ANC control minimizes the feedback signal e(n) after a build-up time. Reference is made to this paper in full extent and especially in respect to the narrowband feedforward control described there (the paper “Active Noise Control: A tutorial review” by Sen M. Kuo and Dennis R. Morgan, published in the Proceedings of the IEEE, Vol. 87, No. 6, June 1999 is incorporated by reference herein in its entirety).y(n)=x(n)wT(n)=w(n)xT(n)=w1(n)sin(ω0n)+w2(n)cos(ω0n).Here, xT(n) designates the transpose of the input wave vector x(n), i.e., the vector at which the columns and rows are transposed.
The vector w(n)=[w1(n), w2(n)] formed from the gain factors is called the phase vector of the ANC control here.
As is shown in FIG. 4B, the gain of the sine waves is adapted by adaptation by means of the phasor vector w(n).w(n+1)=w(n)+μ·conv[s(n), x(n)]e(n),wherein μ indicates the rate of adaptation.
Since the transfer function of the sound generator S(z) is not known to the ANC core for each time, an estimate Ŝ(z) is used, instead, so that the adaptation becomesw(n+1)=w(n)+μ·conv[ŝ(n), x(n)]e(n),wherein ŝ(n) is the pulse response of Ŝ(z). The estimate of the transfer function Ŝ(z) of the sound generator is formed in the known manner. A comparison is made between the signal output from the sound generator with the signal input to the sound generator. Any difference is caused by the manipulation performed to the signal by the sound generator. This manipulation is termed the transfer function of the sound generator S(z). However, the true transfer function is difficult to obtain for complex systems. Therefore, the invention allows for the use of an estimate of the transfer function which essentially compares the signal output from the sound generator (20) with the signal input to the sound generator for multiple operating conditions to form what is termed the estimate of the transfer function Ŝ(z) and could also be termed an optimal or best available transfer function. This includes among other things the effect of a digital-to-analog (D/A) converter, reconstruction filter, power amplifier, loudspeaker, acoustic path from loudspeaker to error microphone, error microphone, preamplifier, antialiasing filter, and analog-to-digital (A/D) converter.
It was demonstrated in the state of the art that under the assumption that    a) the signal d(n) to be superimposed is a simple wave; and    b) the actuator to be used can provide an amplitude ∥u(n)∥≥∥d(n)∥,it is possible to markedly reduce the average (AVG) of the feedback signal e(n): AVG[e(n)FINAL]˜0.
It is emphasized that the above explanations are only examples, and the present invention also includes other known possibilities for generating the signal y(n) outputted by the ANC core.
It is disadvantageous in prior-art systems for actively influencing sound that attempts are made, as a rule, to extensively or fully cancel a noise generated by the noise source. This leads to an extensively high load on an actuator being used, which is thus available for further influencing the sound (in the sense of a sound design) to a very limited extent only.